1. Field of the Invention
The present invention relates to an optical element capable of varying view angles of a screen in display device, and a display device equipped with such optical element.
2. Description of the Related Art
As to screens of display devices, wide view angles are required which are effective in such a case that these screens are observed by a large number of viewers, and in contrast, narrow view angles are required in order that contents of screens are peeped by a third party, namely, various conditions are required for these screens of the display devices in response to use circumstances thereof.
As optical elements capable of narrowing view angles of screens of display devices, such a film that optical absorption layers and transparent layers are alternately arranged in a louver shape (will be referred to as “louver film” hereinafter) has been practically available.
Louver films have been commercially available as product names of “Light Control Film” marketed by 3M Company, or “View Control Film” marketed by Shin-Etsu Polymer Co., Ltd. Louver films have been utilized so as to avoid peeping actions for screens in public-use appliances such an ATMs (Automated Teller Machines), or in mobile appliances such as portable telephones.
In addition to such display devices which limit view angles by employing optical elements, for instance, louver films, other type of display devices capable of arbitrarily switching a view angle between a condition under which the view angle is wide and another condition under which the view angle is narrow have been proposed.
For example, JP-A-5-108023 discloses such a display device equipped with a plane-shaped light source, an optical shutter plate constituted by two polarizing films and a twisted nematic liquid crystal cell arranged between these two polarizing films, and an optical path control plate constructed of a micro lens array, which is provided on a rear plane of a display-purpose liquid crystal cell.
In this display device, since light emitted from the plane light source is limited by the optical shutter plate, a region on which light passes can be formed at an arbitrary position. Also, a positional relationship between the position of the micro lens array and the light passing region changes, so that the optical path is controlled so as to switch the view angles.
As another liquid crystal display device capable of switching view angles, such a display device has been proposed in which a display-purpose liquid crystal cell and a view angle control-purpose liquid crystal cell are arranged between two sheets of polarizing films.
As such a display device, JP-A-10-268251 discloses a control method for a display device in which a twisted nematic liquid crystal layer is provided on a liquid crystal layer of a display-purpose liquid crystal cell, and also, a twisted nematic liquid crystal layer is provided on a liquid crystal layer of a view angle control-purpose liquid crystal cell. In this case, a proper voltage is applied to the view angle control-purpose liquid crystal cell, so that a view angle range during which the contrast ratio thereof becomes larger than or equal to a predetermined value can be changed.
Also, JP-A-9-105958 discloses such a display device. That is, under such a condition that liquid crystal molecules of a view angle control-purpose liquid crystal cell are aligned along a direction parallel to a substrate which constitutes a liquid crystal cell, or under such an isotropic condition by heating liquid crystal, a view angle characteristic similar to that of the conventional liquid crystal display device may be obtained; and in such a case that liquid crystal molecules of a view angle control-purpose liquid crystal cell are aligned along a vertical direction with respect to a substrate, a view angle can be made narrow.
As an optical element (will be also referred to as “view angle limiting element” hereinafter) which narrows view angles of screens of display devices, a louver film has been practically used. The louver film owns a periodic structure in which an optical absorption layer and a transparent layer are alternately arranged.
As a result, the louver film owns the following problem. That is, moire may occur between a pixel array of display elements and the periodic structure of the louver film, so that an image quality is deteriorated. Further, since the louver film is equipped with the optical absorption layer, this louver film owns another problem that a portion of light is absorbed by the optical absorption layer even along a front plane direction, so that transmittance for light is lowered. Also, the louver film basically corresponds to such an optical element which limits transmissions of light along two opposite directions, for example, an upper side and a lower side of a screen, or a right side and a left side of the screen.